The central nervous system (CNS) represents a site of immune privilege. Limiting the potential for immune mediated pathology is thought to be critical for maintained function and longevity of this sensitive tissue especially given its limited regenerative capacity. However, cells of the immune system are found within the CNS under homeostatic conditions. Regulatory T (Treg) cells are a suppressive population of immune cells that act by preventing activation of potentially pathogenic immune responses. These cells function critically under homeostatic conditions in a number of tissues to prevent autoimmune disease and death in humans and mice. We have observed Treg cells within the CNS tissue under homeostatic conditions and herein propose to evaluate their function in this tissue in the steady state and during neuropathogenic infection with West Nile virus (WNV). For this purpose, we will utilize several genetic mouse models developed in our laboratory, which allow for identification, isolation and specific ablation of Treg cells in vivo, as well as models designed to prevent their entry into the CNS. Treg cells within the CNS will be characterized by immunofluorescent labeling of phenotypic markers and flow cytometery and evaluated for expression of molecules essential for their homing to this tissue. Using mice that express the human diphtheria toxin (DT) receptor exclusively in Treg cells, I will use diphtheria toxin to deplete Treg cells and examine CNS pathology histological and via evaluation of effector immune cell phenotype and activity. In addition, the function of Treg cells in the CNS will be examined by specifically precluding their entry into this tissue using mice with Treg cell specific deficiency in CNS homing molecules and transfer of Treg cells from mice lacking CNS homing molecules into T cell deficient mice. Lastly, we have observed accumulation of Treg cells within the CNS of WNV infected mice and it is know that clearance of this virus from the CNS is a T cell dependent process. Thus, the dynamics of Treg cell entry and their phenotype within the CNS of mice infected with WNV will be evaluated and the mechanism of this entry will be interrogated through infection of mice harboring Treg cells lacking in CNS homing molecules. Together, these experiments will characterize the function and mechanism of entry of Treg cells within the CNS under homeostatic conditions and the role for these cells in CNS viral immunity.